The present invention relates to a method of developing latent electrostatic images, and more particularly to a latent electrostatic image developing method in which a latent electrostatic image on an image bearing surface is converted to a visible image with a developer containing toner particles and carrier granules, namely with a so-called two-component developer.
Magnetic brush developing devices or cascade developing devices are known as devices for practicing such a latent electrostatic image developing method. With use of these developing devices, the developing method has actually been performed and has found wide application in the field of electrographic copying machines. The latent electrostatic developing method, as performed by the above-mentioned devices, comprises the following steps (1) to (3):
(1) The developer is forcibly stirred as by a rotating roller (generally having a spiral blade on its periphery) to triboelectrically charge the toner particles in the developer to a polarity suitable for development (usually to a polarity opposite to the polarity of latent electrostatic images).
(2) The stirred developer is fed, by magnetic conveying means, bucket conveyor or like means, to the developing station, where toner particles in the developer are electrostatically attracted to a latent electrostatic image on an image bearing surface. The latent image is converted to a visible image by the electrostatic attraction of the toner particles to the latent image.
(3) The developer used for the development and containing a reduced quantity of toner particles is collected in the developing device. The developer is replenished with fresh toner particles by toner supply means.
The latent electrostatic image developing method described above has various advantages and therefore has found wide use as already stated, whereas the method still involves problems in the durability of the developer, in other words, with respect to variations in the properties of the carrier granules. It is desired to overcome these problems.
These problems will be described below.
With repeated development of latent electrostatic images by the above method, toner particles initially contained in the developer are incessantly replaced by fresh toner particles, while the carrier granules initially contained in the developer are repeatedly used. If the development of latent images is repeated over a prolonged period of time, a substance, so-called spent toner, progressively thermally adheres to the surfaces of the carrier granules, consequently varying the inherent properties of the carrier granules per se. I have confirmed that, of the variations of the properties, the variation of the triboelectrifying ability of the carrier granules (the ability of the carrier granules to triboelectrically charge the toner particles by frictional contact therewith) immediately adversely affects the development of the latent electrostatic image and leads to the following objections.
(a) When a certain type of developer is used for the developing method, the quantity of the toner particles electrostatically attracted to the latent image gradually increases with the repetition of development, also markedly fogging the nonimage areas (the background of the latent image). Additionally toner particles are deposited on the image bearing surface in the form of a trail rearward from the latent image with respect to the movement of the surface relative to the developing device. (For convenience sake, the deposition of the toner particles will hereinafter be referred to briefly as "trailing.") My research appears to indicate that these objections are attributable to a reduction in the triboelectrifying ability of the carrier granules and to the resulting decrease in the amount of triboelectric charges on the toner particles.
(b) When another type of developer is used for the developing method, the quantity of the toner particles electrostatically attracted to the latent image gradually decreases with the repetition of development and that markedly in the solid center portions of the latent image. My analysis appears to reveal that this is attributable to an increase in the triboelectrifying ability of the carrier granules and to the resulting increase in the amount of triboelectric charges on the toner particles.
To solve these problems, it has recently been proposed to form a hard coating as of Teflon over the surfaces of the carrier granules, but the proposal involves limitations on the kind of resin usable for preparing the toner particles and another problem that the coated carrier granules have a relatively low triboelectrifying ability. The proposed remedy therefore still remains to be fully improved.
It has also been proposed to replenish the developer used for development with toner particles of larger size than the toner particles contained in the developer before it is used for development. However the use of toner particles up to 8 .mu.m in mean size generally causes fogging to the nonimage area, whereas toner particles larger than 15 .mu.m in mean size, if used, usually produce a rough toner image, so that it is presently impossible to use toner particles of widely varying sizes. Thus the second proposed method similarly involves problems and still remains to be fully improved before actual use. Although the method has proved satisfactory for application to developers of the type referred to in paragraph (b) above, the method fails to achieve any good result with use of developers of the type (a). And what is worse, it has been found that the method, if practiced with a developer of the type (a), aggravates the aforementioned adverse effects on development.
Accordingly the developer used for the developing method and containing contaminated carrier granules must invariably be discarded for replacement by a fresh developer after the developer has been repeatedly used for developing latent electrostatic images over a prolonged period of time (to be more specific, after making 10,000 to 20,000 copies, calculated as A4 size, with an electrophotographic copying machine presently commercially available and incorporating a magnetic brush development unit), in other words, about the time when the contamination of the carrier granules in the developer is likely to produce a serious adverse effect on development. Such developer disposal and replacement is very cumbersome and uneconomical.